The pattern of failures of constancy provides a kind of signature of the visual software serving constancy. In scenes containing two or more regions of illumination (i.e., almost always) the range of lightness values is compressed relative to the actual range. To exploit this important clue, we measured gamut compression for a row of 5 target squares, of various gray shades, standing in a spotlight (30 X ambient) suspended inside a vision tunnel lined with checkerboard walls. By varying the luminance range of both the 5 squares and the checkerboard walls we produced 6 conditions (n=15, between subjects) used to test 5 stimulus metrics potentially underlying the compression. The amount of compression was predicted by the ratio of highest target luminance to highest checkerboard luminance (equivalent to perceived illumination difference), but not by overall luminance range nor by the formula in anchoring theory nor by two other metrics. Two additional experiments were run to test the hypothesis that the compression results from a lack of information about the spotlight/ambient illumination difference. To exclude any assumption that the ambient illumination applies to the 5 squares, they were moved into a second room and seen through an aperture, with luminances and sizes held constant. Compression was identical, suggesting that border ownership at the occlusion boundary enclosing the squares is not critical. To reveal exactly the spotlight/ambient illumination difference we placed the row within a rectangular beam of light projected onto the far wall of the original room. Rather than reducing the compression, this produced significantly more compression, suggesting that an occlusion boundary segregates frameworks of illumination better than a cast illumination boundary. Overall our results appear to require a major overhaul of anchoring theory, perhaps with both an anchor and a coefficient of scaling established within each framework.